Content stored on web servers may be freely shared by generating a link to the content and distributing the link to users who wish to access the content. For example, a user may have an online web album and may wish to share a particular photograph from the online web album with a few of his friends. To do this, the web server generates a link to the particular photograph which the user then shares with his friends by distributing the link to his friends.
In one case, it is possible for a friend who receives the link to in turn distribute the link to other people who may then view the photograph without any restriction.
This case is undesirable, as the user wishing to share content with others may wish to restrict or control access to the content.